The great increase in the population and land cost has led to increases in height of apartment buildings. In such multistory apartment buildings, the floors are required to have high impact sound insulating performance to avoid a trespass on a privacy of occupants in the room located directly below. If any impact force is applied to the floor, the floor may vibrate and generate impact sounds. Such impact sounds may be divided into two groups, light-weight floor impact sounds produced by occupant activity such as walking, and heavy floor impact sounds produced by sharp transient type impulses such as caused by falling objects or a child who jumps up and down. The former, light-weight floor impact sounds are reduced effectively by laying a carpet on the floor as a finish floor since light-weight impacts may be absorbed by the carpet with ease. It is, however, difficult with the carpet to reduce heavy floor impact sounds. If any sharp transient type impulse is applied on the carpets, most of the heavy impact is transmitted to the floor panel located directly below, which in turn constitutes an impact transmission path into a supporting structural floor such as a concrete slab, resulting in vibration of the floor and generation of floor impact sounds.
In order to minimize the impact sounds transmitted to the room located directly below, extensive efforts have been made and led to development of floating floors with relatively effective impact sound insulating performance. For example, there is a floating floor comprising a buffer layer of glass wool arranged on a concrete slab, a plurality of floor panels arranged on said buffer layer to constitute a floating subfloor, and a finish floor such as carpets or wooden boards. In such a floating floor structure, if any heavy impact is applied to the floor, the impact force is centered on a part of the buffer layer because of the bending deformation of the floor panel located directly below, and then transmitted directly to the concrete slab without being absorbed by the buffer layer. Thus, it is difficult with this floor construction to obtain effective buffering properties.
On the other hand, there has been proposed a floating floor having a construction as shown in FIG. 20 that comprises a buffer layer B arranged on a concrete slab A, supporting members C such as joists arranged in parallel at suitable spaces on the buffer layer B, floor panels D arranged on the supporting members C to form an air layer E between the floor panels D and the buffer layer B, and a finish floor F. In this floating floor, if and heavy impact force P is applied to a point of the finishing floor F, the impact force P is distributed to the neighboring supporting members C and then transmitted to the buffer layer B. The distributed force P.sub.1 is partially absorbed by the buffer layer B, so that the impact force acting on the concrete slab A is broken up, resulting in decrease of the floor impact sounds transmitted to the room located directly below.
Although the floating floor shown in FIG. 20 makes it possible to lower the heavy floor impact sounds to a level which meets the sound insulation class, L-55, specified in JIS A 1419, there is an increasing demand for development of multistory apartment buildings with a further improved floor impact sound insulating performance which satisfies the sound insulation class, L-50 or L-45, specified in JIS A 1419.
However, it is impossible with the above floating floor to provide a floor which will satisfy the sound insulation class, L-50 or L-45. If a heavy impact force is applied to the floor, the floor panel instantaneously produces deformation due to bending as shown in FIG. 20, which in turn generates large flexural vibrations. This flexural vibration varies in vibrating frequency with the size of floor panels and distances between supporting members C. The greater the magnitude of vibration and the longer the duration of vibration, the greater is the vibration transmitted to the concrete slab A. This results in an increase in the magnitude of impact sounds transmitted to the room located directly below.
In addition, the bending deformation of the floor panels causes compression of the air present in the air layer between the buffer layer B and the floor panels D since the supporting members C obstruct free flow of the air, although a part of the compressed air may escape sideways in the air layer. The compressed air in the air layer serves as an air cushion so that a force P' is transmitted to the concrete slab A through the buffer layer B and that the reaction force thereof acts on the floor panels D, resulting in flexural vibration of the floor panels. Thus, the flexural vibration of the floor panels D and concrete slab A are amplified by the forces due to the compression and expansion of the air in the air layer, resulting in increase in floor impact sounds transmitted to the room located directly below.